1. Field of the Invention
The invention relates to a process for the recovery of metals from used nickel/metal hydride storage batteries in which the material is mechanically comminuted and is divided up into at least a coarse fraction and a fine fraction which are treated separately from one another, the fine fraction being dissolved by means of a strong acid, from which solution the metals are separated stepwise by selective precipitation and/or liquid/liquid extraction and recovered.
2. Description of the Related Art
Nickel/metal hydride (NiMeH) storage batteries are regarded as a future alternative in the field of rechargeable cells and have already gained substantial market shares in recent years. High energy data per unit volume and per unit weight make batteries of this type an energy storage system which is interesting for all applications, be it as appliance battery or as traction battery for electric vehicles.
A disadvantage of this high-performance system is, however, that it requires the use of expensive raw materials, which are scarce when measured against the available resources. The most important components used in the NiMeH storage batteries at present are nickel, rare earths and, in particular cobalt, which is regarded as a strategically important raw material and is therefore also subject to large price fluctuations. Nickel and cobalt are the essential elements in the positive electrode. In addition to nickel and cobalt, rare earths, in particular, are constituents of the hydrogen storage alloy in the negative electrode. The present process relates essentially to NiMeH storage batteries with this alloy composition.
In addition to the active electrode materials, which serve directly for the storage of energy, the storage batteries also contain support materials composed of nickel as important material components.
In view of the high material value which a nickel/hydride cell therefore represents even in the no longer serviceable state, and of the scarce resources for these materials mentioned, their recycling is an urgent economic requirement.
The recycling of nickel/metal hydride storage batteries is expected to depend on recycling methods which are already known from the recovery of nickel and cadmium from Ni/Cd storage batteries, for example in accordance with PCT Publication No. WO 92/03853, or from the treatment of a scrap material containing rare earths (in accordance with U.S. Pat. No. 5,129,945).
A process described in German Patent Specification 42 24 884 has the object of recycling used batteries and storage batteries from domestic refuse. Since these accumulate in an unsorted manner and, among the valuable substances, mercury and copper from primary cells, which do not occur in alkaline storage batteries, also have to be considered, this process is complicated and the equipment correspondingly expensive. The initial steps in the development of the scrap material are, however, similar in all cases: the used cells are first mechanically comminuted and separated into various fractions by screening, air classification and magnetic separation. A magnetic fraction essentially comprising the casing scrap contains nickel and iron and can be processed to produce ferronickel. Plastics from the casings, separators and binders accumulate in a light fraction. Finally, a fine fraction is composed of the comminuted actual contents of the cells, the electrode material and electrolyte material. This fraction contains the mass of the substances to be recovered. They are separated stepwise by a wet-chemical treatment, starting with an acid digestion, and generally recovered in elemental form as metal.
The Ni/Cd storage batteries are distinguished from the alkaline nickel/metal hydride storage batteries in principle only by the negative electrode, whose active mass is composed of a hydrogen-storage alloy on a support.
Hydrogen-storage alloys are intermetallic compounds which can be assigned to an AB.sub.5 type by analogy with the simple binary alloy LaNi.sub.5 or to an AB.sub.2 type by analogy with the alloy TiNi.sub.2. Hydrogen-storage alloys of the AB.sub.5 type for nickel/metal hydride storage batteries are frequently composed of 45-55% Ni, 30-35% rare earths (RE), 5-15% Co, 2-5% Mn and 1.4% Al.
In the reprocessing of nickel/metal hydride storage batteries containing AB.sub.5 alloys, an acid digestion by known methods of the fine fraction originating from the mechanical comminution has proved inadequate because it is precisely some of the particularly valuable raw materials, namely the REs, the Ni and the Co, which fail to dissolve. A specific teaching for the complete inclusion, in particular, of the REs is also not to be found in European Patent EP-A-585 701, which relates to a method for the waste disposal of nickel/cadmium or nickel/hydride cells since it does not deal with the type of metal hydride which may be present or its selection.